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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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2 <html>
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3 <!-- This manual is for FFTW
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4 (version 3.3.8, 24 May 2018).
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5
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6 Copyright (C) 2003 Matteo Frigo.
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7
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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9
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10 Permission is granted to make and distribute verbatim copies of this
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11 manual provided the copyright notice and this permission notice are
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12 preserved on all copies.
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13
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14 Permission is granted to copy and distribute modified versions of this
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15 manual under the conditions for verbatim copying, provided that the
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16 entire resulting derived work is distributed under the terms of a
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17 permission notice identical to this one.
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18
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19 Permission is granted to copy and distribute translations of this manual
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20 into another language, under the above conditions for modified versions,
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21 except that this permission notice may be stated in a translation
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22 approved by the Free Software Foundation. -->
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23 <!-- Created by GNU Texinfo 6.3, http://www.gnu.org/software/texinfo/ -->
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24 <head>
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25 <title>FFTW 3.3.8: Caveats in Using Wisdom</title>
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26
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27 <meta name="description" content="FFTW 3.3.8: Caveats in Using Wisdom">
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33 <link href="index.html#Top" rel="start" title="Top">
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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36 <link href="Other-Important-Topics.html#Other-Important-Topics" rel="up" title="Other Important Topics">
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37 <link href="FFTW-Reference.html#FFTW-Reference" rel="next" title="FFTW Reference">
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38 <link href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" rel="prev" title="Words of Wisdom-Saving Plans">
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64 -->
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65 </style>
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66
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67
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68 </head>
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69
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70 <body lang="en">
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71 <a name="Caveats-in-Using-Wisdom"></a>
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72 <div class="header">
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73 <p>
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74 Previous: <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" accesskey="p" rel="prev">Words of Wisdom-Saving Plans</a>, Up: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="u" rel="up">Other Important Topics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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75 </div>
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76 <hr>
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77 <a name="Caveats-in-Using-Wisdom-1"></a>
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78 <h3 class="section">3.4 Caveats in Using Wisdom</h3>
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79 <a name="index-wisdom_002c-problems-with"></a>
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80
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81 <blockquote>
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82 <i>
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83 <p>For in much wisdom is much grief, and he that increaseth knowledge
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84 increaseth sorrow.
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85 </i>
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86 [Ecclesiastes 1:18]
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87 <a name="index-Ecclesiastes"></a>
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88 </p></blockquote>
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89
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90 <a name="index-portability-1"></a>
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91 <p>There are pitfalls to using wisdom, in that it can negate FFTW’s
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92 ability to adapt to changing hardware and other conditions. For
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93 example, it would be perfectly possible to export wisdom from a
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94 program running on one processor and import it into a program running
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95 on another processor. Doing so, however, would mean that the second
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96 program would use plans optimized for the first processor, instead of
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97 the one it is running on.
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98 </p>
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99 <p>It should be safe to reuse wisdom as long as the hardware and program
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100 binaries remain unchanged. (Actually, the optimal plan may change even
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101 between runs of the same binary on identical hardware, due to
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102 differences in the virtual memory environment, etcetera. Users
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103 seriously interested in performance should worry about this problem,
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104 too.) It is likely that, if the same wisdom is used for two
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105 different program binaries, even running on the same machine, the
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106 plans may be sub-optimal because of differing code alignments. It is
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107 therefore wise to recreate wisdom every time an application is
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108 recompiled. The more the underlying hardware and software changes
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109 between the creation of wisdom and its use, the greater grows
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110 the risk of sub-optimal plans.
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111 </p>
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112 <p>Nevertheless, if the choice is between using <code>FFTW_ESTIMATE</code> or
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113 using possibly-suboptimal wisdom (created on the same machine, but for a
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114 different binary), the wisdom is likely to be better. For this reason,
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115 we provide a function to import wisdom from a standard system-wide
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116 location (<code>/etc/fftw/wisdom</code> on Unix):
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117 <a name="index-wisdom_002c-system_002dwide"></a>
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118 </p>
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119 <div class="example">
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120 <pre class="example">int fftw_import_system_wisdom(void);
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121 </pre></div>
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122 <a name="index-fftw_005fimport_005fsystem_005fwisdom"></a>
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123
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124 <p>FFTW also provides a standalone program, <code>fftw-wisdom</code> (described
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125 by its own <code>man</code> page on Unix) with which users can create wisdom,
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126 e.g. for a canonical set of sizes to store in the system wisdom file.
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127 See <a href="Wisdom-Utilities.html#Wisdom-Utilities">Wisdom Utilities</a>.
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128 <a name="index-fftw_002dwisdom-utility"></a>
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129 </p>
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130 <hr>
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131 <div class="header">
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132 <p>
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133 Previous: <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" accesskey="p" rel="prev">Words of Wisdom-Saving Plans</a>, Up: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="u" rel="up">Other Important Topics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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134 </div>
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135
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136
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137
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138 </body>
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139 </html>
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